I don't understand which processor is better and why - intel i7 or xeon e5. Not fully understand how can I upgrade my memory in future... As sample, what if my first RAID1 pack is fullfiled, I'll need to buy 2x HDDs to make second RAID1 pack? Or can I simple buy HDD and just put it in this gdm machine??

All in all, I want to understand with what troubles can I meet in the future and I want to be prepared... But for know I need fast and furious %)) killing machine. Below I copy-paste my chose of configuration (maingear.com), Please write what do you think about it, what would you change and also what do you think about maingear?

This is my budget, and I want to build a workstation which wouldn't become brainf**n at least in 4 years.

Just because you have it doesn't mean you should spend it. ANY SYSTEM you build today will most likely be serving you well four years from now. However, any system you build today will be considered old tech and out-dated two years from now. This is regardless of how much you spend today.

Unless you're running a dozen or so virtual machines or doing high math scientific calculations, there is no reason to go with a multi-cpu system. Any workstation class Core I7 CPU will more than suit your needs now and a few years down the line. Xeon processors are more for server applications, though they can be used for workstations, whereas a CoreI7 processor is geared more towards everyday applications.

Concerning RAID arrays: RAID arrays are pretty much set once they are defined. Expanding a RAID array would be a function of the RAID card (or motherboard). If expanding the RAID array is something you think you'll have to do in the future, then look for a RAID card that allows you to do so. Additionally, you have a chassis with three hot-swap SATA drive bays. That just screams RAID 5. Look into it.

Concerning RAM: The amount of RAM you have is determined by what motherboard you have. What motherboard you have determines how many RAM slots you have and what RAM configurations you can have. For instance, a motherboard with a max RAM limit of 64GB and 4 RAM slots, you could have a RAM configuration of 4x16GB (64 GB), or 2x16 (32 GB) or 4x8GB (32 GB) of RAM. As long as you're under the motherboard's max RAM limit, you can add more.

However, how much RAM you need is going to depend on what applications you're using. Since you stated the main purpose of this system is for art, then I would assume the files you are working with are fairly large. Simple equation. Take the largest file size you're working with and double it. That's how much RAM you should have installed. Only if you're working with 32GB files should you even consider 64GB of RAM. For most people, 16GB of RAM is more than enough and anything more is wasted.

Concerning your build:Case: I like the chassis, in particular the front mounted 3 Drive SATA hot-swap.Motherboard: Probably drop down to a more standard LGA2011 socket motherboard.CPU: Drop down to a more standard Core I7 LGA2011 processor.RAM: Unless you're working with larger than 8GB images, 16 GB is more than sufficient.GPU: Start with one GPU. Not even sure you can run two workstation GPUs simultaneously.OS Drive: No complaintsStorage Drives: Go for three (or more) drives in a RAID 5 array.RAID: Most motherboard come with some sort on software RAID. I'd recommend going with a hardware RAID card (8 ports).Power Supply: This system, even with dual workstation graphic cards would never need more than 850 watts (even running 24/7).Optical Drive: Assuming you're backing up data to this drive, no complaints.Sound Card: Unless you're an audiophile, there is absolutely no need for a sound card.Network: If there is any possible way to have this system hard-wired to a network, skip the wireless connection.